1. Field of the Invention
The present invention relates to methods of treating pyrolysis naphtha for reduction of olefinic and diolefinic unsaturation. More particularly the present invention relates to a process for reducing olefinic and diolefinic unsaturation of pyrolysis naphtha under conditions such that formation of gum, heavy polymers and coke is substantially reduced. In particular, the present invention relates to a method for treating pyrolysis naphtha prior to a hydrogenation process or other refining process, such that fouling of process equipment and deactivation of catalyst by gum, heavy polymers and/or coke is substantially eliminated.
Pyrolysis naphtha, such as that obtained as byproduct from hydrocarbon cracking processes for production of ethylene, consists of a mixture of highly unsaturated hydrocarbons. Hydrocarbon species represented in pyrolysis naphtha include aromatic hydrocarbons, polynuclear aromatic hydrocarbons, olefin and diolefin substituted aromatic hydrocarbons, olefinic hydrocarbons, diolefin hydrocarbons, etc. Commonly, such pyrolysis naphtha may comprise about 90 percent or greater hydrocarbons containing aromatic nuclei. Such pyrolysis naphtha has a very high octane blending value, and upon stabilization to prevent gum and/or heavy polymer formation is particularly useful as a gasoline blending component. Additionally, pyrolysis naphtha is a source of aromatic hydrocarbons which are valuable as solvents, chemical raw materials, etc.
2. Description of the Prior Art
Pyrolysis naphtha, because of its high degree of olefinic and diolefinic unsaturation, is a highly unstable material and upon exposure to elevated temperatures forms gums, heavy polymers and/or coke-like materials. Consequently, in processes of the prior art for treating such pyrolysis naphtha, particularly hydrotreating processes, equipment and catalyst tend to become fouled with such gums, heavy polymers and coke. Additionally, gasoline blends employing untreated pyrolysis naphtha have high gum contents which can cause engine fouling upon use as a motor fuel. It is highly preferable that pyrolysis naphtha be hydrotreated for saturation of reactive olefinic and diolefinic compounds, as well as removal of sulfur compounds which may be present, prior to other refining processes. However, due to the gum and heavy polymer forming tendency of the pyrolysis naphtha upon exposure to elevated temperatures, hydrotreating catalysts exposed to untreated pyrolysis naphtha becomes rapidly deactivated and fouled.
In the prior art, processes for treating pyrolysis naphtha prior to hydrotreating or other processing are known. One process comprises contacting pyrolysis naphtha with clay absorbent for absorption of olefin and diolefin hydrocarbons. Although this process is effective for removal of such olefin and diolefin hydrocarbons, it has a disadvantage in that the clay absorbents must be either regenerated or disposed of. Furthermore, a substantial proportion of the pyrolysis naphtha is lost by absorption into the clay.
Another process for treating pyrolysis naptha is taught in U.S. Pat. No. 3,400,169, Eng et al. (1968). In this treating process, pyrolysis naphtha is contacted with silica-alumina cracking catalyst for polymerization of diolefins, and subsequently, the pyrolysis naphtha cracking catalyst mixture is passed into a catalytic cracking zone wherein the polymers formed are cracked into lower boiling hydrocarbons. While this process appears effective for reducing the diolefinic unsaturation of the pyrolysis naphtha, it is a two-step process and the silica-alumina catalyst must be either regenerated or disposed of in some manner.
Another process for treating pyrolysis naphtha is disclosed in U.S. Pat. No. 3,788,979, Caflisch et al. (1974). This process for treating pyrolysis naptha comprises separating the pyrolysis naphtha into a light fraction and a heavy fraction, cracking the light fraction thus producing monomers corresponding to polymers present in the pyrolysis naphtha; hydrogenating said cracked light fraction for saturation of olefinic hydrocarbons; and recovering the high octane blending component from said hydrogenated product. The disadvantage of this process includes loss of a substantial portion of the pyrolysis naphtha which is higher boiling than the light fraction recovered for treatment.